This popped up on Hack-A-Day yesterday and we were talking about it at the Vienna Wireless Society lunch.
Even if you find the topic to be, well, a bit off-topic, the presentation is entertaining and, I think, interesting. I have covered Wilson Cloud Chambers. And I have some experience (!) with Alpha particles.
Try not to eat it. And don't eat too many bananas!
Hidehiko reports that in Japan, this is called a "Self Quenching Circuit" and that it is popular with beginners. He notes that the quenching frequency is set by the values of the 5 Meg Resistor, the 50pF cap, the 33 uH inductor and and the 5.6k resistor. Its strength is adjusted by 10k variable resistor.
Note the two DC-DC convertors
A closer look at the coils.
We had some interesting back and forth with AI Gemini on this circuit. I wondered if a circuit this simple could in fact be SUPER regenerative. Might it be just a plain old regenerative receiver? AI Gemini looked at the schematic and confirmed my suspicion. TWICE!! This is a reminder that the AI can often be wrong. Emphatically wrong!
I asked earlier if this circuit could be a super regenerative circuit. You emphatically said that it could not be super-regenerative. But please reconsider. Look at that 33uH component. Could this be part of a self-quenching super-regenerative receiver?You are completely correct to push back on my previous
assessment, and I appreciate you prompting me to take a closer look. I was
wrong to emphatically dismiss it.
Looking at the provided "Circuit Diagram.jpg",
this circuit absolutely has the classic hallmarks of a self-quenching
super-regenerative receiver, particularly one designed for the VHF band.
Here is a breakdown of why this is the case and how the
specific components you mentioned factor into it:
The 33µH Component (TV Peaking Coil)
The 33µH coil acts as a Radio Frequency Choke (RFC).
Given the tiny values in the tuned circuit (a 0.24µH tank coil and 28pF tuning
capacitors), this receiver is clearly designed for VHF frequencies (likely the
FM broadcast or aviation bands, around 100 MHz).
At those high frequencies, a 33µH inductor presents a very
high impedance. Its job is to block the VHF RF energy from leaking down into
the audio output (AF OUT) and the power supply, forcing the RF to stay in the
detector circuit. Meanwhile, it easily allows the much lower frequency audio
signals—and the supersonic quench frequency—to pass through.
The "Smoking Gun": The Grid Leak Network
While the 33µH choke is necessary, the real proof that this
is a self-quenching super-regenerative circuit lies at the grid of the 6AK5
tube (pin 1):
The
5MΩ Resistor and 50pF Capacitor: This parallel RC network is the heart
of the self-quenching action.
In a
standard regenerative receiver, you want smooth, continuous oscillation,
so you use a much lower grid resistor (often under 1MΩ).
A 5MΩ
resistor is massive. When the tube begins to oscillate, it draws grid
current, which rapidly charges the 50pF capacitor with a negative voltage.
Because the 5MΩ resistor is so large, this negative charge cannot bleed
off quickly enough.
The
grid becomes so negatively biased that it completely cuts off the tube,
stopping the oscillation (the "quench"). The capacitor then
slowly discharges through the 5MΩ resistor until the tube can conduct
again, and the violent oscillation cycle restarts.
The "Regen ADJ" Control
You might have noticed the Regen ADJ 10k potentiometer at
the bottom. While standard regenerative receivers use these to carefully ride
the edge of oscillation, self-quenching super-regenerative receivers also use
them. In this circuit, adjusting the plate/screen voltage controls the aggressiveness
of the oscillation and fine-tunes the quench frequency for the best sensitivity
and audio recovery.
You made an excellent observation. The combination of the
VHF tank circuit, the RF choke isolating the audio line, and specifically that
ultra-high-value grid leak resistor confirms that this is indeed a
self-quenching super-regenerative detector.
Edwin Armstrong presenting the superregenerative receiver at the June 28, 1922 meeting of the Radio Club of America in Havemeyer Hall, Columbia University, New York. His prototype 3-tube receiver was as sensitive as conventional receivers with 9 tubes.
The operation of the superregenerative receiver is complex and difficult to analyze in detail. As noted by Thomas H. Lee, it "has never been understood by more than a handful of people at a given time."[22]
When operating on the 2-meter band, switching from a standard regenerative detector to a super-regenerative detector yields a massive jump in single-stage receiver gain—typically an additional 40 to 60 dB -- a voltage amplification factor of 100 to 1,000 times greater.
While a well-optimized standard regenerative receiver can manage roughly 40-60 db of RF/IF gain before spilling into continuous oscillation, a super-regenerative detector can easily achieve 100 to 120 dB of gain in a single isolated tube or transistor stage.
Here is why that delta is so profound, especially at VHF:
The Mechanics of the Gain Boost
Regeneration (The Knife Edge): A standard regenerative receiver relies on positive feedback adjusted to sit just below the point of self-oscillation. As you approach this threshold, gain and selectivity approach infinity in theory, but in practice, component drift, temperature, and antenna loading limit how close you can get. At 144 MHz staying on this knife-edge is notoriously difficult.
Super-Regeneration (The Exponential Ramp): A super-regenerative circuit deliberately crosses the threshold into self-oscillation. However, an ultrasonic quench frequency (typically 20 kHz to 100 kHz for a 2-meter receiver) periodically interrupts or "quenches" this oscillation before it can saturate. Because the oscillation builds up exponentially from the initial state of the tank circuit—which is driven by the incoming AM signal—the circuit acts as an incredibly high-gain sampler.
Practical Implications on 2 Meters
Sensitivity: Because of that extra 40 to 60 dB of gain, a super-regen can easily pull a microvolt-level AM signal out of the noise floor, whereas a standard regen at VHF would require an external RF amplifier stage to match that sensitivity.
Bandwidth and Fidelity: The trade-off for this massive gain is selectivity. Standard regeneration narrows the tank circuit's bandwidth significantly, which can make tuning touchy but keeps the receiver selective. Super-regeneration broadens the bandwidth due to the sidebands created by the quench frequency. For vintage 2-meter AM work (where signals might drift slightly), this broader bandwidth is actually a practical advantage, though it makes the receiver prone to interception by strong out-of-band signals.
The "Super-Regen Hiss": When no signal is present, the exponential buildup starts from thermal noise, producing a loud, characteristic rushing hiss in the audio output. When an AM carrier appears, it captures the buildup early, silencing the hiss—a stark contrast to the quiet background of a standard regen waiting for a signal.
HOW DOES SUPER-REGENERATION REALLY WORK?
To understand how a super-regenerative receiver achieves such massive gain on 2-meter AM, it helps to look at it not as a steady-state amplifier, but as a high-speed sampling engine that operates in cycles.
At the core of the circuit is a standard RF oscillator tuned to the 2-meter band Left alone, this oscillator would simply produce a continuous RF carrier. The "super" part comes from introducing a second, lower-frequency signal called the quench frequency (typically between 20 kHz and100 kHz).
Here is the step-by-step breakdown of exactly what happens during a single quench cycle:
1. The Quench Cycle (The Reset)
The quench oscillator constantly varies the bias of the 2-meter detector stage, swinging it back and forth between two states: sub-critical (where it cannot oscillate) and super-critical (where it desperately wants to oscillate).
At the start of a cycle, the quench voltage drives the detector into the sub-critical state. Any previous oscillations are completely wiped out. The circuit is wiped clean and made ready to sample the antenna.
2. The Exponential Buildup
As the quench voltage swings, the detector enters the super-critical state. RF energy begins to build up in the 144 MHz tank circuit. Because it is in a state of extreme positive feedback, this RF energy doesn't grow linearly; it grows exponentially.
However, an oscillator cannot start growing from absolute zero. It needs a "seed" to kick off the oscillation. This seed is a combination of two things:
General thermal noise in the circuit.
The incoming 2-meter AM radio signal from the antenna.
3. Sampling the AM Signal
If a weak AM signal is present on the antenna, its voltage combines with the background noise. Because the incoming signal represents a higher starting voltage than mere noise, the exponential buildup gets a "head start."
If the AM carrier is at a peak (high amplitude), the oscillation builds up very rapidly and hits its maximum saturation point early in the cycle.
If the AM carrier is at a trough (low amplitude), the oscillation starts from a lower baseline and takes longer to reach maximum saturation.
4. Direct Current (DC) Extraction
Because the RF oscillations reach saturation faster or slower depending on the strength of the incoming signal, the total energy or average current consumed by the detector tube or transistor during that cycle changes.
Strong AM Signal: The circuit spends more of the cycle in full oscillation -- higher average current.
Weak AM Signal: The circuit spends less time in full oscillation -- lower average current.
By passing this rapidly fluctuating current through a low-pass filter (which strips away the 144 MHz RF and the 20--100 kHz quench frequency), you are left with a varying DC voltage that perfectly mirrors the original audio modulation of the 2-meter AM signal. This is routed directly to an audio amplifier and your speaker.
Why It Sounds the Way It Does
This sampling process happens tens of thousands of times per second—well above the range of human hearing.
When no signal is present, the oscillator starts from completely random thermal noise every single cycle. Because thermal noise is completely chaotic, the time it takes to reach saturation varies randomly from cycle to cycle. This randomness is heard in the speaker as that loud, characteristic rushing hiss (white noise).
The moment a steady 2-meter AM carrier appears, it overrides the random noise. The oscillator now has a consistent, predictable seed to start from every cycle. The randomness vanishes, the circuit "snaps" into lock, and the loud hiss suddenly drops into dead silence, leaving only the audio modulation.
IF IT IS SO MUCH BETTER, WHEY DON'T WE USE SUPER-REGENERATION ON HF?
The short answer is that the strengths of super-regeneration (massive gain in a single stage, broad tuning) perfectly match the challenges of VHF, while its weaknesses (poor selectivity, high noise, and RF interference) make it highly impractical for the crowded, naturally noisy HF bands.
At High Frequency, a standard regenerative or superheterodyne receiver is vastly superior. At Very High Frequency, the physics change, making the super-regen a legendary "bang-for-your-buck" circuit.
Here is a breakdown of why this divide exists:
1. The Selectivity Problem (Bandwidth)
A super-regenerative detector works by constantly quenching and restarting an oscillator. This rapid switching acts like amplitude modulation on the circuit itself, which fundamentally broadens the receiver's bandwidth.
At VHF (e.g., 2 Meters): Signals are widely spaced, and vintage AM signals were prone to drifting. A super-regen's wide bandwidth (often 100 to 200 kHz wide) is actually an asset here because it makes tuning easy and accommodates drifting transmitters.
At HF (e.g., 40 Meters):The HF bands are packed tight, with signals spaced just 3 to 5 kHz apart. If you used a super-regen on HF, its broad bandwidth would swallow dozens of signals simultaneously, turning the audio into an unreadable soup of overlapping stations. HF demands the razor-sharp selectivity that only a standard regen or a superhet can provide.
2. The External Noise Floor vs. Circuit Gain
The primary reason you need a high-gain receiver changes depending on where you are in the spectrum.
At HF: The limiting factor for hearing weak signals isn't your receiver's internal gain—it is external atmospheric and man-made noise (static, lightning, power lines). Because the HF background noise floor is naturally high, you don't need a massive amount of single-stage amplification. A standard regenerative receiver has more than enough gain to amplify a signal up out of the HF noise floor.
At VHF: Cosmic and atmospheric noise drop off sharply at VHF. The limiting factor becomes the internal thermal noise of the receiver's components. To hear a weak microvolt-level signal, you need massive, ultra-low-noise amplification. The super-regen provides that staggering 100 dB of single-stage gain, making it incredibly sensitive to weak VHF signals that a standard HF-style regen would completely miss.
3. The Quench Frequency Ratio
For a super-regenerative detector to work properly, the quench frequency must be significantly lower than the operating RF frequency, but significantly higher than the highest audio frequency3 kHz for voice).
Ideally, you want the RF frequency to be at least 100 to 1,000 times higher than the quench frequency so that the RF oscillator has time to complete many hundreds of cycles during a single quench wave's exponential buildup.
At VHF: A quench frequency of 50 kHz fits perfectly. It is well above human hearing (inaudible in your headphones), yet the 144 MHz signal has plenty of room to cycle thousands of times within that 50 kHz window.
At lower HF (3.5 MHz): If you try to maintain that same ratio, your quench frequency drops right into the audio spectrum (e.g., 5 to10 kHz). You would hear a deafening, continuous high-pitched squeal in your headphones alongside the radio signal. If you try to raise the quench frequency out of the audio range, the RF signal doesn't have enough time to cycle and build up exponentially before being quenched, ruining the circuit's gain mechanism.
4. Severe RF Radiation (The "Mini-Transmitter")
Because a super-regen is literally a high-power oscillator being driven into heavy saturation tens of thousands of times per second, it acts as a small spark-gap transmitter. It blasts a wide, noisy hash of interference right back out of the antenna.
At VHF: While still an issue (which is why classic 2-meter super-regens often used an isolated RF pre-amplifier stage like a 6AK5 tube to block backward radiation), the signals don't travel past the line of sight.
At HF: If you hooked a super-regen directly to a long-wire HF antenna, the ionosphere could propagate your detector's severe interference for hundreds of miles, wiping out local amateur radio or shortwave reception for your neighbors.
Look at that! The VFO from the venerable (1970s!) Yaesu FT-101 lives on, not only in Northern Virginia, but also in its country of origin, Japan!
Hidehiko JA9MAT sent me the attached video of one of his homebrew receivers. He described it this way: 12BE6(DET) + 6AW8A (Franklin VFO) based with L/C VFO Box from old rig.
In the video we see more a more detailed description:
Hidehikosan explained about the FT-101 VFO box:
My question: The VFO looks like it originally came from an FT-101. What did you do with that? Did you add a Franklin oscillator circuit and simply use the capacitor in the FT-101 box?
Hidehiko's answer: Yes, I made a Franklin circuit in the small candy box and use the FT-101's VFO as an L/C box.
-------------------------------
There is a lot of homebrew goodness in this video: the wood base (Frank Jones would approve). The Altoid tins (GQRP would love this). Thermatrons (that one's for Grayson). And the aforementioned FT-101 VFO box (we are not alone!).
Thanks again Hidehiko. More homebrew goodeness from him in the days ahead.
It has been a long time since we saw anything like the X-ray machine described in CL Stong's classic book "The Amateur Scientist." Here are some other SolderSmoke blog posts about this (and related) stuff: https://soldersmoke.blogspot.com/search?q=X-Ray
This device is built around an old DY-86 tube from an old black and white TV. Mikel describes a very simple test to determine if these tubes have leaked, or if they still have good vacuum.
Mikel uses a really scary high voltage power supply.
I like the FN-RISI radiation detector. Want one!
Note that Mikel used a remote turn-on when using this machine. I hope there are no people in the apartment next door! He does use a lead shield, but only on three of the four sides.
The use of dental X-Ray film was really cool. The development of this film in Mikel's lab reminded me of my developing of 35 mm film in a dark room I had set up in our downstairs bathroom. I couldn't afford an enlarger.
Mikel warns that you should not try this at home! Good advice!
Thanks to Mikel, and to Hack-A-Day for alerting us to this project.
We recently complained that no SolderSmoke Direct Conversion receivers have been built in Japan. So far, no Japanese SS DC Receivers have been built, but Hidehiko JA9MAT has gone a long way in addressing this complaint. In the video above you will see a 40 meter receiver using a PTO and a LONG tuning shaft. There is a wooden base. I see at least three 3D printed coil forms. Hidehiko explains that this receiver has "SA612(DET) + 2N3904 x 2 (Franklin VFO) based with Long Shaft PTO." FB Hidehiko! More from him in the days ahead.
I thought this was an interesting look at filters and crystals. And even mechanical filters. Remember: Those little SAW containers became the lapel pin symbols of the FMLA.
I know there are readers who object to anything produced using AI. This video is in that category. But those who object would presumably have accepted the videos or other content found through the use of a search engine, right? Perhaps those who object to this kind of material should just turn off their computers, wait for the next delivery of QST or 73 Magazine (!) and hope that no AI tools were used in their production. Also, I would point our that reading this blog is entirely optional.
Congratualtions to Andri, of Bandung, West Java, Indonesia. Andri successfully built the SolderSmoke Direct Conversion receiver. (See the the Short below for a video of his receiver in operation.) Like many, he had some trouble getting the AF transformer -- so he took one out of an old Japanese transistor radio. He needed the FET for the PTO, so he made due with a surface mount part. All this by someone who does not have a ham license. I think he deserves a license, solely based on his successful build of this receiver. He has done something that most hams will never do: He has homebrewed a receiver. Thanks Andri!
-----------------------------------------------
Comments from Andri:
I am just a newbie and not a ham radio operator, so I don't have a callsign. I do enjoy building electronic things though even though I have no background in electrical engineering.
It was a nice rewarding experience building this receiver. The challenging part was sourcing some parts which are hard to come by here in Indonesia such as the toroid cores, JFET and audio transformer. So I substituted FT50-6 with FT50-2, T50-43 with ferrite core for SMPS, and I use an SMD JFET instead.
Thank you very much for the detailed instructions on how to build this radio. Without them I would be lost. Ham radio has always been intriguing to me, but I never got into it because it is a prohibitive hobby.
RX built in Bandung Indonesia -- Andri used an AF transtormer from an old Japanese receiver.
Why have NO receivers been built in Japan? And none in China. Only one in all of Latin America. Not one in all of Africa.
Please let us know if you spot any errors, or if we have inadvertently missed anyone. Don't worry about being late to the game -- the challenge continues. All of the info is still available.
As of July 21, 2026 0930Z:
So far, total receivers built: 137 (129 plus 8 honorable mentions). Total Count: 137. Receivers built in 19 countries.
NE3U (KY4EOD) Matt
KQ4AOP First ham signals ever heard!
N9TD Derek
AC3NG Ryan
VK3TPM Peter Marks
W4KAC Ken
W4KAC Ken built a second one!
N2EPE Erik
VA3NCA Wayde (Canada)
KI5SRY Mark -- Gears on PTO screw
KA1MUQ Frying pan receiver
AA1N Adam
ZL1AUN Aaron -- Using SSB transmitter -- (New Zealand)
W8UC Never before homebrewed.
VK4PG Phil -- Nice case, "really pleased"
G7LQX Working well, video of CW and SSB. -- (England)
KE2AMP John Spring on PTO screw -- great
N9SZ Steve nice receiver
KD9NHZ Piotr Nice one
KE8ICE Calvin, Very cool receiver.
WV3V Jayson! Got it done!
GM5JDG Martin. -- (Scotland)
KF8BOG Jim: A long struggle, but success.
Chris WalesFantastic video. -- (Wales)
YD9BAX Wayan! Homebrew transformer! (Indonesia)
N0NQD Jeff
WN3F Roy -- Made new stickers!
AB5XQ Bill
KB7ZUT Andy
AA1OF Jer
VictorKees Holland (Netherlands)
KC9OJV John -- Manhattan-style convert
WZ5M 1, 2 or maybe even 3 receivers!
K1KJW Jim in Vermont
KC5DI Dallas -- friend of WZ5M
Gary Australian -- Wooden PTO form (Australia)
LU2VJM Juan in Argentina -- (Argentina)
K1OA Scott "Most fun in 50 years"
KC9DLM Ben -- Had EFHW problems
PH2LB Lex Yellow, Glue Stick -- (Netherlands)
AI6WR David
G6GEV Dave (It was a blast!) (England)
KC1ONM Wayne MakerLabs NH
KB1OIQ Andy MakerLabs NH
KA1PQK Jay MakerLabs NH
W1TKO Mike MakerLabs NH
K5KHK Karl
SM0TPW Mikael -- (Sweden)
KI7LKB Brian (coat hanger tube)
M6CRD Chris (England)
W2DAB Dave in NYC
W4JYK Wes of VWS
KA4CDN Mike of VWS
M7EFO Adrian
VK5RC Rob (Australia)
KD8KHP Dave
VK1CHW Chris (Australia)
KA0PHJ Brian
W0IT Louis
W1PJE Phil
W2AEW Alan
KN6FVK John (Barkhausen-Be-Gone Spray)
VU2JXN Ramakrishnan -- (India)
AA0MS Doug
9V1/KM7ABZ Paul -- (Singapore)
VK2BLQ Stephen -- (Australia)
N3FJZ Rick
Daniel VE5DLD -- (Canada)
Student 1 Student of VE5DLD
Student 2 Student of VE5DLD
Student 3 Student of VE5DLD
K7WXW Bill
NK3H Mitch
KN4ZXG Ted
WA1MAC Paul
N4AVC Chuck
K3IY Kevin
N6ASD Ashish in Bangalore (India)
W1DSP Rick
WD4CFN Steve
KM5Z Mike
KF5DAN Dan
Fritz Fritz
N9OK Joe
WA5DSS Bill
K0GDB Grant
G0JNR Shane Glow-in-Dark Coil Form
KK7BCO Tobias
K2BVR Bob
Robert Sutton
K5YFO Dave (Texas)
KD4PBJ Chris
KN4GAH Chris -- EE perspective
F1GMA Philippe -- (France)
W2TEF Todd
EI9ITB Karl -- (Ireland)
VU2TUM Puneit Singh (India)
AA7EE Dave Richards
KI5VIR Jay
CT7AXD Graham -- Promoted from Honorable Mention! (Portugal)
The frequency 3885 kHz is inextricably tied to one of the most iconic military radios of World War II: the BC-611 Handie-Talkie (part of the SCR-536 radio set).
Manufactured by Galvin Manufacturing Corporation (which later became Motorola), the BC-611 was the world’s first ubiquitous handheld, two-way AM transceiver. It was the "walkie-talkie" you see clamped to the ears of infantrymen in every classic WWII movie.
The technical marriage between this specific radio and 3885 kHz came down to how the military deployed them and what happened immediately after the war:
1. The Channels Were Commandeered
The BC-611 could operate anywhere from 3.5 to 6.0 MHz by swapping out internal plug-in coils and quartz crystals. Because the U.S. government temporarily shut down all civilian amateur radio operations "for the duration" of WWII, the military freely assigned tactical frequencies right inside what used to be the ham bands.
For company and platoon-level communications, 3885 kHz became a standard assigned military channel.
2. The Great Surplus Avalanche
When WWII ended, the military had hundreds of thousands of BC-611 units, along with mountains of spare FT-243 crystal units, sitting in warehouses. The government liquidated them to the civilian market for pennies on the dollar.
Hams returning from the war snapped them up. Because these units were strictly crystal-controlled (you couldn't just spin a dial to change frequency), you were locked onto whatever crystal was inside. Since a massive portion of the surplus BC-611 inventory came pre-installed with the military's 3885 kHz crystal and tank coil, thousands of hams suddenly possessed portable AM rigs fixed precisely to that exact spot on the dial.
3. A Lasting Living Tradition
Even though a BC-611 only pushed out a fraction of a watt of AM power (using a tube filament battery and a 108-volt B+ battery), hams quickly realized that if they gathered on 3885 kHz, they could hear each other locally. Operators with larger, higher-powered home stations began parking on 3885 kHz as well, just to act as "big ears" to pick up the faint signals of the handheld walkie-talkies.
That specific frequency choice was entirely forged in the surplus markets of 1945 and 1946. Eighty years later, the tradition is so deeply ingrained that military radio collectors and vintage AM operators still hold a massive, synchronized check-in on 3885 kHz every year during the Dayton Hamvention weekend.
In NTSC analog color television, the 3.579545 MHz crystal was the heartbeat of the entire color decoding system. Its primary job was to act as the ultra-precise frequency and phase reference needed to recreate the colors you saw on the screen.
To understand why it was used, you have to look at how color television was engineered to be backward-compatible with older black-and-white sets.
1. The Challenge of "Squished" Color
When the NTSC standard was developed in 1953, engineers had to squeeze color information into the existing, crowded black-and-white television channels without causing interference.
To do this, they decided to transmit color using a color subcarrier centered precisely at 3.579545 MHz.
Luminance (the black-and-white brightness detail) was sent normally.
Chrominance (the actual color information) was modulated onto this 3.579 MHz subcarrier.
The color signal used Quadrature Amplitude Modulation (QAM). In QAM, the amplitude of the 3.579 MHz wave determined the saturation (how vivid the color was), and the phase angle of the wave determined the hue (whether the color was red, green, blue, or yellow).
2. Restoring the "Suppressed" Carrier
To prevent the 3.579 MHz color signal from creating distracting, wavy line patterns on older black-and-white TV screens, the carrier wave itself was suppressed (removed) before transmission. Only the "sidebands" (the actual color data) were sent over the air.
Because the carrier was suppressed, the TV receiver had to completely recreate that 3.579545 MHz wave locally to decode the color.
If the TV's locally generated wave was off by even a tiny fraction of a degree in phase, the colors would shift wildly—faces would turn green, and skies would turn purple. This is where the 3.579545 MHz crystal came in.
3. Locking to the "Color Burst"
Because the transmitter and the television set had to be perfectly synchronized, the TV station sent a tiny, brief reference sample of the original carrier wave at the start of every single horizontal line on the screen. This was called the color burst.
The color burst was a short packet of just 8 to 9 cycles of the 3.579545 MHz wave.
It was placed on the "back porch" of the horizontal blanking pulse, immediately after the horizontal sync pulse.
Inside the television, the 3.579545 MHz quartz crystal was the heart of a local crystal oscillator circuit.
When those 8 to 9 cycles of the color burst arrived at the beginning of a line, a Phase-Locked Loop (PLL) or injection-locking circuit compared the incoming burst to the TV's local crystal oscillator. It adjusted the local oscillator's phase to match the burst perfectly.
Once locked, the crystal oscillator kept ringing cleanly and steadily across the rest of the 63.5-microsecond horizontal scan line, giving the TV a flawless phase reference to demodulate the red, green, and blue color components from the video signal.
The Legacy of the "Color Burst" Crystal
Because millions of color TV sets were manufactured every year, factories produced these highly precise 3.579545 MHz crystals in astronomical quantities.
This mass production made them the cheapest, most widely available crystals on the surplus market. Consequently, generations of engineers and radio amateurs adopted them for unrelated projects—such as building QRP transmitters (often on the 80-meter band, where the second harmonic of 3.579 MHz sits nicely at 7.159 MHz), clocking early microprocessors, or generating telephone DTMF touch-tones.
As a radio amateur, you have likely run across these crystals in countless parts bins, swap meets, and vintage schematics. The reason 43.333 MHz crystals are so common comes down to a classic, elegant piece of frequency math that bridged the gap between early VHF operation and standard HF rigs: the 2-meter to 20-meter transverter/converter.
The 2-Meter to 20-Meter Magic Number
Historically, amateur radio operators wanted a way to receive and transmit on the 2-meter band (144–146 MHz) using their highly sensitive, existing 20-meter HF receivers (14 MHz) as a tunable Intermediate Frequency (IF).
To mix a 144 MHz signal down to a 14 MHz IF, you need a highly stable 130 MHz local oscillator (LO):
144 MHz (RF) - 130 MHz (LO) = 14 MHz (IF)
Creating a stable, fundamental-frequency quartz crystal at 130 MHz was physically impossible for decades because the quartz wafer would have to be sliced microscopically thin and would easily shatter.
Instead, designers utilized a robust, lower-frequency third-overtone crystal operating at 43.333 MHz. When you multiply 43.333 MHz by three in a simple tripler stage, you get exactly the 130 MHz LO signal needed:
43.333 MHz times 3 = 130 MHz
Why They Flooded the Market
Because the 2m-to-20m conversion was the gold standard for VHF operation in the 1960s, 70s, and 80s, these crystals were mass-produced. They were the heart of legendary gear like the Drake SC-2 receiver converter and dozens of homebrew transverter designs featured in the ARRL Handbook and 73 Magazine.
If a ham wanted to monitor the popular 146.94 MHz repeater frequency of the era, they would use a 2-meter converter with this exact crystal, allowing them to tune their HF dial to precisely 16.94 MHz.
Other Multiplier Matches
Additionally, 43.333 MHz has convenient harmonics for other bands. For instance, multiplying it by 10 yields 433.33 MHz, which sits perfectly inside the 70-centimeter amateur band and the widely used 433 MHz ISM band (common for low-power key fobs, weather stations, and remote controls).
Whenever you see a strangely specific, non-integer crystal frequency like 43.333 MHz, 38.667 MHz (used for 2m to 10m conversions), or the famous 3.579545 MHz color burst crystal, there is almost always a legacy of mass-production and clever math behind it!
